Modulated power supplies, such as Pulse Width Modulated (PWM) power supplies, are widely used in a variety of applications. In a PWM power supply a power switching device, such as a power transistor, is turned on and off at a high frequency, with the width of the ‘on’ periods varying in sympathy with the amplitude of a modulating input signal. The resulting train of output pulses from the switching device is smoothed by a low pass filter to deliver a supply voltage which varies in sympathy with the modulating input signal.
A PWM power supply can have a single phase or multiple phases, with the contributions of individual phases summing to provide an overall output. Multi-phase PWM power supplies have an advantage over single phase PWM supplies in that they can deliver better resolution in the time domain and increased current. It should be noted that the term ‘phase’ relates to apparatus which receives an input signal and operates a switching device rather than a phase in an electrical sense.
One known application of a modulated power supply is in supplying power to a power amplifier. An envelope of the signal which is to be amplified is used as a modulating signal for the power supply and the resulting, modulated, power supply signal is fed to the power amplifier. In this way, the power supply signal follows the envelope of the signal to be amplified and the efficiency of the power amplifier can be improved.
In a pulse width modulated system, there are several constraints. The sampling rate for each phase, i.e. the rate at which the amplitude of the modulating signal is sampled, must be at least twice the highest frequency of the modulating signal to avoid aliasing effects. This imposes a lower limit on the sample rate. The switching devices have a finite frequency range over which they can be operated before they begin to exhibit non-ideal behaviour, and this determines the maximum rate at which the switching devices can be turned on and off. A combination of the sample rate and maximum switching rate determine the range of discrete pulse widths that a coding unit of the PWM supply can assign to the switching signal. When the modulating signal has a wide bandwidth this forces the sampling rate to be high and results in a limited range of coding values for the PWM coder. This limits the resolution, in terms of amplitude, of the final output signal. As an example, the base stations in a third generation, two channel Universal Mobile Telecommunications System (UMTS) are expected to transmit and receive signals having a bandwidth of around 10 MHz. Assuming a sampling rate of 20 MHz and a maximum switching rate of 160 MHz, this only allows the PWM coder to have a set of eight different coding values (pulse widths). Thus, the amplitude of the output signal, at any point in time, can only be resolved to one of eight different values.
Accordingly, the present invention seeks to improve the performance of a modulating power supply particularly, but not limited to, situations where the modulating signal has a wide bandwidth.